Silver halide crystals have been the dominant photosensitive material in photographic processes for more than a century. During this time, improvements in sensitivity have produced a broad range of materials with specialized photographic properties. Modern photographic emulsions consist of a very large number of tiny silver halide grains suspended in a polymeric matrix, typically gelatin. Such emulsions are prepared with silver chloride, bromide, or iodide, or with mixtures of these halides. When light of the appropriate wavelength, strikes the silver halide grains, a latent image is formed which corresponds to the visible image that appears upon photographic development.
The preparation of a photographic element generally includes the steps of precipitation, sensitization, and coating. The photographic properties or overall sensitivity of an emulsion are dependent upon several variables which may be controlled at various steps in the photographic process. Factors which influence sensitivity include the composition (proportion of halides), and the average size and morphology (shape) of the grains. The morphology of emulsion grains varies widely with the conditions of precipitation. In the precipitation step, grains of an emulsion are formed by mixing, in the presence of a protective colloid, solutions of a soluble silver salt and of one or more soluble halides. The method, rate and conditions of this precipitation step control, in large part, grain structure, size and distribution.
Some emulsions also require the presence of other substances in the precipitation solutions. For example, U.S. Pat. No. 3,320,069 issued May 16, 1967 to Illingsworth, describes an emulsion with low internal sensitivity prepared by precipitation or pre-washing treatment of the silver halide with thiocyanate ions
The shape of the grains tends to vary with composition. Silver chloride grains, for example, are usually cubic, while silver bromide grains are cubic, octahedral or cubooctahedral. In the formation of the latter, the boundary between cubic and octahedral depends, in large part, on the silver ion concentration of the precipitating conditions, generally reported as pAg (-log [Ag.sup.+ ]). Typically, cubic grains form at a lower pAg than octahedral grains The presence of iodide increases the probability of forming grains with octahedral faces, and shifts the boundary to a lower pAg. At a fixed pAg, the grain shapes are progressively more octahedral as the amount of iodide in the emulsions is increased. See generally "The Theory of the Photograph Process," T. H. James, ed., 4th Ed., Macmillan Publishing Co., Inc. (1977) p. 94.
Sensitizers used in the sensitization step of the photographic process include sulfur-containing agents, noble metals, reducing agents and polymeric agents. Spectral sensitizers may also be added to make the silver halide grains more sensitive to longer wavelengths of light.
After sensitization, certain additives are used to prepare the emulsion for coating. For example, surfactants are added to facilitate wetting and spreading of the emulsion of the support. Tetraazaindenes are added to reduce spontaneous development in unexposed regions, and aldehydes can be used to permit high temperature processing.
Pressure fogging is a persistent problem with many silver halide photosensitive materials. Pressure exerted on an silver halide emulsion can generate electrons through a mechanism not completely understood. Emulsion grains, similar to other inorganic crystals and crystallites, have crystal defects such as dislocations, and sufficient stress can generate mobile electrons within the grains. Such stresses can be induced by poor camera design, such as squeezing roller pairs or other guides, mishandling of film by folding or twisting, or other physical phenomena which stress the film prior to development. The silver halide grains cannot discriminate between pressure-induced electrons and photon (or light-) induced electrons. Consequently, pressure-induced fog often occurs as lines in a negative which resemble scratches.
Pressure fog is a response to applied stress that fogs (i.e., makes developable in a non-imagewise fashion) some fraction of the emulsion grains. Such pressure-fogging can occur, and degrade the photographic performance of the film, at any point in the film's use up until development. Pressure-fogging does not require any imagewise exposure to be detectable, but if such an exposure should occur, the effects of pressure-fogging will be apparent as areas in the image with abnormally high density (in the negative).
Pressure fogging is distinctly different from pressure desensitization. The latter requires an exposure to be detectable. The application of stress to the film prior to exposure damages some fraction of the grains such that imaging efficiency is seriously degraded. This loss of efficiency in the stressed region translates to a diminished density (desensitization) in an exposed region of the film.
Several attempts have been described in the prior art that attend to the problem of pressure sensitivity. U.S. Pat. No. 4,177,071, issued Dec. 4, 1979 to Debrabanders et al., discloses radiographic emulsions substantially insensitive to formation of pressure marks upon rapid processing which consist of silver halide grains of diameter of at least 259 nm and hydrophilic colloid to silver halide ratio of 1.0. U.S. Pat. No. 4,495,277 issued Jan. 22, 1985 to Becker et al. discloses emulsions with surface-sensitized grains having a core/shell structure that have improved behavior with respect to pressure, when tested by applying a pressure trace to the emulsion immediately after the beginning of development.
Still other prior art has called for the addition of certain compounds to avoid the effects of pressure. U.S. Pat. No. 4,247,620 issued Jan. 27, 1981 to Nagatani et al. describes a silver halide photographic material for use in high-contrast photography which is resistant to pressure as measured by a folding of the film test by incorporating a quaternary ammonium, phosphonium or arsenium compound in the photographic material.
Japanese Patent 62-018538 reports pressure resistivity of an emulsion which includes thiocyanate. Japanese Patent 59-050438 discloses an emulsion with improved pressure properties which includes heterocyclic nitrogen and tellurium compounds. Japanese Patent 61-22641 describes an emulsion which has an anti-pressure property and is prepared from an ammonium compound as the silver halide solvent. Despite attempts to provide photographic emulsions which maintain photographic speed and developability, yet control pressure-fog, the art has not responded with a photosensitive material having features that adequately address these needs.